A Fresh Look at Galaxy Scaling Relations and What They Reveal About Galaxy Formation

Despite the apparent complexity of galaxies, some of their most basic properties (mass M, binding energy E, angular momentum J, etc) obey simple power-law scaling relations. Such relations provide some of the clearest insights into galaxy formation. This talk focuses on the relations between circular velocity V (a proxy for specific energy, e = E / M ~ V^2) and mass M and between specific angular momentum j = J / M and M. The observed forms of these scaling relations for the stellar components of galaxies, Ms ~ Vs^4 and js ~ Ms^0.6, are closely linked to the corresponding relations for their dark halos in CDM-type cosmologies, Mh ~ Vh^3 and jh ~ Mh^(2/3). We have obtained new kinematic (H_alpha and HI) and photometric data that extend the relations for late-type galaxies (spirals and irregulars) over the full range of stellar masses: Ms ~ 10^8 to 10^12 Msun. From the same data, we derive the “retention factors” for mass fM = Ms / Mh (aka stellar-to-halo mass ratio, SHMR) and specific angular momentum fj = js / jh, finding fM ~ Ms^0.4 and fj ~ 1 independent of Ms. In contrast, for the most massive early-type galaxies (ellipticals and lenticulars), fM declines with Ms and fj << 1. We draw some simple but important conclusions from these empirical results about the physical origins of several basic properties of galaxies, including their sizes, compositions (stars vs gas), morphologies (disk vs spheroid), and feedback drivers (stars vs AGN). We also show that the observed forms of fM and fj are reproduced in contemporary cosmological hydrodynamical simulations of galaxy formation (IllustrisTNG).